Is there any way to prove who paid for the bong? Does it show the last four numbers of the credit card or anything? Or can you prove which URL it was ordered from? There's got to be some way to show your parents the truth.

The “best” response is to let her know what she did worked. That your folks freaked out, and she got you good. Then wish her well in the future, and say you hope she meets a nice guy soon. This will drive her nuts thinking you are the good guy that got away.